Process for cementing oil wells



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COATING OR Pl AQ C Patented June 2, 1942 moss REFERENCE PROCESS FORCEMENTING OIL WELLS Gordon 1). Patterson, Wilmington, Del., assignor toE. I. du Pont de Nemours & Company, Wilmington, Del., a corporation ofDelaware No Drawing. Application December 18, 1939, Serial No. 309,899

4 Claims.

This invention relates to the transference of plastic material which ispredominately solid through passageways of 'relatively smallcrosssection, and particularly to a process for improving the flowcharacteristics of cementitious material required to be transferredthrough passageways of relatively small cross-section, and moreparticularly to a process for introducing into oil wells concretemixtures suitable for forming filters for petroleum, and to acomposition of matter suitable for forming filters for petroleum andhaving improved fiow characteristics.

The need for oil well filters appears to be generally recognized. Manywells being completed today are equipped with devices for straining sandfrom crude petroleum. The fracture of sandstone and other softformations by drilling or by explosives intended to increase the yieldof oil leaves pulverized material which may subsequently be carried outof the well if the flow of oil is rapid, and not infrequently thismaterial, through its abrasive action, damages the well casing or otherequipment. Numerous expedients have been employed for removing sand fromcrude oil, among which might be mentioned screens, perforated liners,and gravel packs, all of which serve as filters in the well. However,filters of porous concrete, because of their low cost, wide permeabilityrange and absolute contact with the oil-producing formation, are nowbeing given considerable attention.

1n constructing filters of this latter type, one method is to introduceconcrete into an oil well by confining the concrete in plastic state inthe well casing by plugs or pistons situated above and below the chargeand forcing this assembly to a predetermined depth where an opening inthe casing permits discharge of the concrete.

The employment of this method for introducing porous concrete into wellsrequires that the concrete be sufiiciently plastic or fluid to flowthrough the well casing. Unfortunately, concrete which is fluid whenfreshly mixed is not always porous when set, and concrete which is bothfluid and porous may be lacking in other essential properties so thatfor filter purposes it is useless. Porous concrete prepared by gaseousinflation of a sand-cement-water slurry is, under some circumstances, anexample of material of this type. will be developed if such concretesets under the usual atmospheric pressure, the conditions of temperaturewhich prevail in the deeper wells and the pressures under which cappedwells are maintained have been found to be sufficiently Although porestructure high to promote collapse of the pore structure of inflatedconcrete during the setting period.

The type of porous concrete most satisfactory for constructing filtersfor petroleum is that obtained by using a major proportion of aggregatewith a minor proportion f binding agent. A

filter of this composition has essentially the structure and porosity ofthe mass of aggregate alone since the binding agent is disposedprincipally at the points of contact of the aggregate particles. Amongthe advantages of this type of filter cement, are: (1) resistance toextreme pressure which would promote collapse of filter cement renderedporous by inclusion ofgas bubbles, and (2) the control which may beexercised over porosity of the filter by proper choice of aggregate andproportion of aggregate to binder.

Concrete in which porosity is thus regulated by the size ofaggregate'and the aggregate-cement ratio employed in its production, isquite suitable for the preparation of oil-well filters in most respects,but lacks fluidity in the un-set condition. An aggregate-cement ratiohigh enough to give a structure consisting essentially of aggregateparticles cemented to one another at their points of contact yields arigid filter which is extremely resistant, at all times to compressiveforces, but the resultant concrete slurry is not amendable to transferthrough steel oilwell casing. Friction between the casing and thepebbles and the opposing pressure of fluid confined within the wellinduces the slurry to lock in a rigid mass which can be moved only byextreme pressure.

It is an object of this invention to render concrete of highaggregate-cement ratio, i. e.

cdlfcl'e't'm con isting principally of aggregate (or analogous plasticmasses consisting predominantly of solid material), transferable throughsteel pipe or other type of passageway so that the cementing of oilwells, and especially those which are maintained under temperatures andpressures which render inflated concrete useless as a material forconstructing filters, may be accomplished readily with aggregate-type ofconcrete mixture which is suitable for the construction of porousconcrete structures such as oilwell filters, and which has improved flowcharacteristics, A still further object is an improvement in flow ofplastic high-aggregate-types of porous concrete. These and other objectswill more clearly appear in the description which follows.v

These objects are achieved according to this car invention by adding tothe concrete mixture con; taining aggregates of suitable size and insuitable relative proportion for the intended purpose, suflicient inert,water-immiscible diluent liquid to fill the voids which exist naturallyin pgrous concrefi of high aggregate-cement ratio. This liquid serves asa pressure-transmitting fluid and further serves to lubricate theconcrete mass, with the result that its fluidity is increased to thepoint where it may be transferred through a conduit or pumped into awell by application of only slightly more force than is required toovercome the opposing pressure of the fluid confined in the well. Theadditional fluidity imparted to aggregate-type porous concrete byinclusion of inert, water-immiscible liquid also aids materially in itsplacement in the catity of the well which is to be cemented; theconcrete flows into irregularities and fissures more readily thanunmodified material and thereby seals off the oil-producing formationmore effectively.

The invention is illustrated by the examples which follow. Parts are byweight.

The following two examples illustrate the improvement of the presentinvention (Example II) over the prior art practice (Example I).

Example:

Concrete prepared from -raded agreate (3-8 mesh and a density of aou 5)Fortland cement 125, and water 80 parts was confined in a 2" steel pipeby steel pistons equipped with rubber facings. It was found thatconcrete so confined exhibited extreme resistance to fiow. A column only5" in length required a pressure of from 800 to 1200 lbs/sq. in. toinitiate fiow downward through the pipe and movement of the column couldbe maintained only if a pressure of about 200 lbs/sq. in. wascontinually applied.

Erample II Graded aggregate,(3-8 mesh and a density 0 a ou or an cement150, and water 75 parts were mixe oroug y. Suflicient mineii oil (about195 parts) was then added to the concrete mass to provide a slightexcess above that required to fill the interstices in the concrete. Whenthis oil-modified concrete was tested for fluidity, a 5 column could epumped downward through the 2" steel pipe by the application ofpressures amounting to only a few pounds per square inch, substantiallythe pressure required to overcome the friction effect due to the steelpistons and rubber gaskets.

Examplelll This example illustrates that the presence of oil neitherinterferes with setting nor reduces the strength of the concrete aftersetting.

Graded aggregate (3-8 mesh and a density of a ou 00, Portland egg ent125, and water 76 parts were mixed until uniform. Two hundred ten partsof light mineral oil were added to this slurry, and mixing was con inueduntil a uniform composition was obtained. The 1; modified concrete and asimilar concrete which i7ars iiot Yfiodified were cast in separate moldsand were observed from time to time during setting. Upon standing for ashort time the pebbles-which had been separated by a film of oil wereobserved to become united at their points of contact by means of thePortland cement paste, and not only did this concrete set at the normalrate but it was comparable in strength to the unmodified material of thesame composition.

An oil-well in which the oil-producing cavity after shooting is roughlyspherical and about five feet in diameter is cleaned by one of the usualprocedures, e. g. bailing or circulation, preparatory to cementing. Acolumn of water is maintained in the casing to prevent a blowout, andconsequently it is convenient to force the porous concrete slurry intoposition while confined be tween plugs. About three tons of freshconcrete are then prepared from materials which are combined in theratio: aggregate (3-8 mesh and a density of about 1.5 0, ortland cement125, water 80, and crude oil (Saybolt Universal viscosity, 30 sec. at 0parts by weight. The crude oil is not added to the mixture until thewater-cement paste is uniformly distributed throughout the aggregate.The freshly prepared slurry is then charged into the casing (8"diameter) following a piston which fits the casing tightly but yet isable to move freely. When all the concrete slurry has been put into thecasing, a second piston is placed in the casing following the charge andthe entire assembly is pumped to the bottom of the well where theconcrete is discharged. At the end of 36 hours the porous concrete plugis partially drilled out to provide a sump for receiving the petroleumwhich filters through. When drilling equipment is not at hand or ifdesired the sump may be formed by lowering a form or plug into the welland forcing it into the porous concrete while it is still in the fiuidcondition. After the concrete has set the form is removed. In some casesthe form may be placed before the cement is introduced.

It should be understood that the method of placement which has just beendescribed is usually preferred for introducing concrete into wellsmaintained under pressure because it utilizes oilwell cementingequipment which is widely available. introducing the porous concreteinto the well may be employed. The concrete can be transported throughthe head of water to the oil-producing cavity in a carrier or travelingbucket. The well can also be pumped dry and sealed under pressure. Inthis event the porous concrete can be introduced into the casing bymeans of an air-lock and permitted to fall freely to the bottom of thewell where its flow characteristics permit uniform distribution.

Example";

The viscosity of the inert material may in some instances become afactor in determining how-readily it is transmitted through pipes. Whenthe flow of material is downward a less viscous diluent may be used thanwhen the concrete is to be pumped horizontally or upward. In factviscosity of the diluent may be varied within wider limits when thecharge is to be pumped downward. When flow is desired in anotherdirection, however, a more viscous diluent is more satisfactory. Toobtain an oil of more suitable viscosity it is of advantage in certaincases to modify it with a thickening agent such as the paraifin used inthe following example.

A concrete slurry prepared from 3-8 mesh gravel 1000, Portland cement126, and water 76 In particular instances other methods for- -Examiner 1P S, A %%S P E LASUC CROSS REFERENCE ture of concrete and diluent notonly flows downward through a pipe very readily but can be pumpedhorizontally or upward with about the same facility.

It is to be understood that the above examples are for purposes ofillustration only and that in the matter of proportions and materialsthe invention is susceptible to wide variation, particularly as follows.

The classification aggregate is considered to include all types of solidinert materials which might be included in a filter structure to furnishbulk and to produce voids by its inability to form a solid structure.Among the commonly available substances which are quite useful in thisrespect are crushed stone coke, gravel, furnace cinder, pumice, marble0515s, an as furnace slag, although other materials may e used. For mostpurposes particles retained by a No. 8 sieve, i. e. particles not lessthan about inch in diameter (or greatest dimension) are more suitablefor production of filters of permeability greater than 20 darcies. Thereis no upper limit at which aggregate becomes unusable except that itmust be small enough to pass through the steel pipe which will conductthe filter terial.

PortlanQ or other hydraulic cement, because s 'cheapness andavailability, is used as a preferred binding agent, but other inorganiccements such as laster of l?aris, slaked lime mortar, magnesium oxchloride cement, puozolamc cement or alurmnous cemen can S'e H use also.We prefer Wise minding agents in most cases because of their lower costand relative indifierence to water or crude oil in oil wells. Organicbinding agents such as thermoplastic resins@ viscose, urea-formajFeTTH'man, neoprene latex, or or amc soli'it'i'o'ns' of film-forming materialssuch as nitro- "Ellulose or cellulose acetate may be used; nowever,treatment of the binding medium with a suitable coagulant after thefilter medium has been placed in position may be required with certainof these binding agents. The ratio of binding agent to aggregate willdepend upon the degree of permeability desired of the filter and uponthe degree of fineness of the aggregate, but it is to be noted that anysize of aggregate. and any ratio of aggregate to binder may be used inthe practice of this invention, provided the mixture is such as toproduce a concrete structure of the desired porosity and strength onsetting. To state the matter more concretely, it may be said that anymixture may be used with advantage in the process of this inventionprovided the said mixture is capable on setting up to produce a filterof permeability greater than 20 darcies, which permeability is ingeneral attained using aggregate sized from about to about /2" andaggregate-cement volume ratios within the range of from about 3 to about20, and preferably from about 5 to about 10.

The nature of the diluent fluid is not critical except for therestriction that it be immiscible with the binding agent solution orsuspension and that it be chemically inert in presence of the bindingagent phase. If the binding agent is of the aqueous type such asPortland cement or a cement like plaster of Paris, the pressuretransmitting fluid may be a petroleum-type product such as crude orpartially refined petroleum, or a product of the refinement of petroleumsuch as asoline kerosene, petrolatum,

es of any or all? these 5 oi s, fish and other animal oils and chlorinae with each other or with araffin wax. If desired, however, otherimmisciblebrganic liquids or low melting solids may be used for the samepurpose. Among such compounds are vegetabl h drocarbo If, on e otherhand, an organic material is to be used as the binding agent, thediluent fluid selected in any given case will depend on the physicalstate of the binder. For example, if water solutions of viscose orurea-formaldeh de resin are used, Wat 6rimmisci e o1s 6f''6r'g'a'n1ciquids of the type mentioned above may be used as diluents, while watermay be used as the pressure-transmitting fluid if cellulose acetate ornitrocellulose or a synthetio res n inorganic solvent'fs'used as the 111ng agen The amount of diluent fluid which is added is not criticalproviding enough is used to fill the pores existing in the mass ofaggregate and binder. The mixture will not have the desired fluidityunless the minimum limit is observed. In practice the minimumrequirement of diluent is readily ascertained by adding it to the massof aggregate and binder in a suitable container such as a concretemixer. The pore-filling diluent flows into the mass readily until theinterstices have been filled and then will appear as a layer coveringthe mass as excess diluent is added. It is not necessary to add diluentbeyond this point.

This invention provides a process whereby the porous type of concretemost suitable for use in constructing oil well filters, namely, the highaggregate-cement ratio type of concrete, may be easily introduced intothe well cavity without injury to the well casing and without requiringspecial apparatus in addition to that at present used for introducingnon-porous concrete into oil wells. It also provides a' means forsimilarly handling and placing concrete of this type wherever needed forstructural or other purposes.

The invention is not limited to the specific embodiments hereindisclosed, it being understood that all obvious variations andequivalents falling within the spirit of the invention are to beincluded within the scope thereof as defined by the following claims.

I claim:

1. A process for improving the flow characteristics of a porous concretemixture comprising a major portion of aggregate and a minor proportionof binding agent without increasing its porosity which comprises addingas a constituent of the concrete mixture, without stable emulsification,an inert liquid, which is immiscible with the binding agent, in anamount not less than that required to fill the voids existing in saidconcrete mixture.

2. A process for improving the flow characteristics without increasingthe porosity of a porous concrete mixture comprising aggregate, water,and Portland cement in proportion to set up to a porous mass ofpermeability greater than 20 darcies, which comprises adding to saidmixture, without stable emulsification, a petroleum-type liquid in anamount not less than that required to fill the voids existing in saidconcrete mixture.

3. In the process of forming a concrete filter in an oil well cavitywherein an un-set concrete composition comprising a major amount ofaggregate and a minor amount of binding agent is passed down theoil-well easing into the oil-wellcavity and is then allowed to set, thestep which comprises adding as a constituent of the conbetween movablepistons in an oil well casing and pressure is exerted on the uppermostpiston to force the plastic mass down to the desired point in the well,the improvement which comprises adding as a constituent of said plasticmass, without stable emulsification, an inert liquid which is immisciblewith the binding agent and which is present in an amount not less thanthat required to fill the voids existing in said plastic mass.

GORDON D. PATTERSON.

